Tetratopic Phenyl Compounds, Related Metal-Organic Framework Materials and Post-Assembly Elaboration

ABSTRACT

Disclosed are tetratopic carboxylic acid phenyl for use in metal-organic framework compounds. These compounds are useful in catalysis, gas storage, sensing, biological imaging, drug delivery and gas adsorption separation.

This application claims priority benefit of application Ser. No. 61/195,876 filed Oct. 10, 2008, the entirety of which is incorporated herein by reference.

This invention was made with government support under grant no. DE-FG02-01ER15244) awarded by the Department of Energy. The government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to tetracarboxylic acid and related species. These compounds can be used in a crystalline metal-organic framework. More specifically, the invention relates to a tetratopic phenyl and related metal-organic framework compounds. Such metal-organic framework compounds of the present invention are suitably used in catalysis, gas storage, sensing, biological imaging, drug delivery and gas adsorption separation.

BACKGROUND OF THE INVENTION

Crystalline metal-organic frameworks (MOFs) comprise a rapidly growing class of permanently microporous materials.¹ They are characterized by low densities, high internal surface areas, and uniformly sized pores and channels. These properties point to a broad range of potential applications, including chemical separations,² catalysis,³ gas storage and release,⁴ biological imaging,⁵ and drug delivery.⁶ Many of these applications require comparatively large cavities. On the other hand, MOF syntheses typically produce catenated structures, thereby reducing cavity size, increasing density, diminishing vapor-uptake capacity and diminishing gravimetric surface area (See FIG. 1). In most cases non-catenated MOFs are desired, but experimentally catenated structures are often obtained.

Optimal performance in applications depends upon the ability to obtain MOFs having: a) cavities and pores of optimal size, shape, and/or chirality, and b) interior and/or exterior surfaces of suitable chemical composition. Systematic (i.e. predictable) tunability of pore size and, to some extent, surface chemical composition, has indeed been nicely demonstrated for certain families of MOFs.⁷ For others, however, even minor changes in synthesis conditions or strut composition can lead—seemingly unpredictably—to significant differences in cavity-defining metal-node/organic-strut coordination and/or degree of framework catenation.⁸ Additionally, certain desirable functional groups may be difficult to incorporate directly into MOFs, either due to thermal instability under materials synthesis conditions⁹ or because of competitive reaction with intended framework components. Together, these complications can make direct assembly of MOFs that are optimal for specific applications particularly challenging.

An emerging alternative design strategy is to construct robust precursor MOFs and then chemically elaborate their internal and/or external surfaces to impart desired properties. While only a handful of examples has thus far been reported,^(3f,4e,10) it is clear that the strategy is a powerful one. For example, Wu and co-workers added highly catalytic Ti(IV) sites to the chiral dinapthol-based struts of a pre-formed MOF and subsequently used the MOF to facilitate the enantioselective addition of ZnEt₂ to aromatic aldehydes.^(3f) Kaye and Long^(10e) photochemically attached Cr(CO)₃ to a benzene dicarboxylate strut in η⁶ fashion. Wang and Cohen^(10a) were able to modify IRMOF-3 post-synthetically by reacting pendant amines with anhydrides; they subsequently demonstrated that modification could alter the affinities of a simple cubic MOF for various guest molecules.^(10c) Various other efforts have been directed to: a) the introduction of charge-compensating alkali metal cations (potential H₂ binding sites¹¹) via strut reduction,^(4e,10g) b) surface tailoring of nonporous metallo-salen MOFs via reversible coordination of salen metal sites with chiral ligands and subsequent use of the modified MOFs to accomplish partial separation of the R and S forms of 2-phenylethylalcohol,¹² and c) “click” based modification¹³ of alkyne-bearing struts to impart hydrophilicity.^(10f)

SUMMARY OF THE INVENTION

The present invention can be directed to a class of metal-organic framework building blocks comprising tetratopic carboxylic acids and related compounds. Such a building block can feature a phenyl ring core, substituted at the 1-, 2-, 4-, and 5-positions with substituted phenyl ring spacers. The carboxylic acids, used to bind metal ion or cluster nodes, can be located at the 4-position of each phenyl ring spacer, although other positioning can be employed.

Without limitation, this invention can be directed to a broad range of tetracarboxylic acid and related species; e.g., 4′,5′-bis(4-carboxyphenyl)-[1,1′:2′,1″-terphenyl]-4,4″-dicarboxylic acid (named according to ChemDraw Ultra 12.0; other names include 4,4′,4″,4′″-benzene-1,2,4,5-tetrayl-tetrabenzoic acid or 1,2,4,5-tetrakis(4-carboxyphenyl)benzene, 2. (See, e.g., scheme 1, below; and, more generally and without limitation, structural variations of the sort provided in FIGS. 2A-2C.) Without limitation, deprotonated 2 would, as: a) an unusually shaped molecule, resist formation of catenated MOFs, b) a tetra-topic building block, produce robust frameworks, and c) a nonplanar moiety, potentially produce a 3D framework. Such characteristics can favor the formation of comparatively large cavities which can be shown experimentally. Additionally, representative of various other embodiments of this invention, 2 can readily form more complex, non-catenated MOF materials when combined with other candidate organic struts. In accordance with this invention, various other compounds can be prepared using synthetic techniques of the sort described herein, or straightforward modifications thereof, as would be understood by those skilled in the art made aware of this invention. (see, e.g., FIGS. 3, 4, and 5).

Nearly all approaches taken in the prior art entail elaboration of struts of intact framework compounds¹⁴ (and, see the recent report by Hwang et al.¹⁵). In contrast thereto, this invention involves MOF cavity modification via activation^(4f,16) and elaboration of framework nodes. As shown below, cavity modification can affect material ability to sorb molecular hydrogen.

With respect to one non-limiting embodiment, a robust, non-catenated, and permanently microporous metal-organic framework (MOF) material has been synthesized by combining a new nonplanar ligand, 4,4′,4″,4′″-benzene-1,2,4,5-tetrayl-tetrabenzoic acid, with a Zn(II) source under solvothermal conditions. The new material features cavities that are readily modified via activation and functionalization of framework nodes (as opposed to struts). Preliminary investigation of the “empty cavity” version of the material and six cavity-modified versions reveals that modification can substantially modulate the MOF's internal surface area, pore volume, and ability to sorb molecular hydrogen. Regardless, any metal source can be selected that would favor the formation of a comparatively large cavity to produce a MOF with a broad range of potential applications. Accordingly, as would be understood in the art, a metal site component can, without limitation, comprise another metal ion capable of coordination chemistry comparable to or available through Zn(II).

Other objectives, features, benefits and advantages of the present invention will be apparent from this summary and its descriptions of certain embodiments of such MOF compounds, and will be readily apparent to those skilled in the art having knowledge of the synthetic techniques described therewith. Such objectives, features, benefits and advantages will be apparent from the above as taken into conjunction with the accompanying examples, data, figures and all reasonable inferences to be drawn therefrom.

Accordingly, the invention can also be directed to a gas adsorption separation process characterized by adsorption separation of components in a gas by contacting the gas with a MOF of the invention. Such a process can be employed to reduce the emission of gases from industrial processes. Specifically, the MOFs of the instant invention can be used for the adsorption of such gases as, for example, H₂, CO₂, N₂ and CH₄. In a certain embodiment, the MOF of the invention acts as an adsorbent with high selectivity for one or more gases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a A) noncatenated structure and a B) catenated structure.

FIGS. 2A-C represent ligands of the sort useful in accordance with this invention.

FIG. 3 is an example of a noncatenated MOF with a large cavity that can be used for hydrogen, carbon dioxide and methane storage and/or separation.

FIG. 4 is another example of a noncatenated MOF with a large cavity that can be used for hydrogen, carbon dioxide and methane storage and/or separation.

FIG. 5 is an example of a noncatenated MOF with a large cavity that can be used for catalysis.

FIG. 6 depicts crystallographically derived MOF 3: (A) structure of 3, (B) topology and connectivity of 3, (C) ac-plane, looking down b-channels, and (D) ab-plane, looking down c-channels (coordinated DMF molecules are shown in space-filling fashion, while non-coordinated solvent molecules (disordered) are omitted from the structure representations).

FIG. 7 depicts (A) thermogravimetric analyses of 3 as synthesized (black), 4 (red), and 4 resolvated (blue); and (B) first-derivative thermogravimetric analyses plots for solvent-evacuated, py-R-modified MOFs. For presentation clarity, curve for MOF modified with 8 is omitted.

FIG. 8 shows the ¹H NMR of dissolved 3′ (top), 4 (middle), and 4+5 (bottom).

FIG. 9 depicts the isotherms for uptake of H₂ at 77K and 1 atm. by: 4+9 (bottom), 3′ (middle), and 4 (top).

FIG. 10 depicts H₂ uptake versus pore volume (red, open squares) and surface area (blue, diamonds).

FIG. 11 depicts the simulated (bottom) and “as synthesized” bulk (top) powder x-ray diffraction patterns for 3.

FIG. 12 shows the ¹NMR spectra in D₂SO₄/D₂O of 3′ (bottom) and 4 (top).

FIG. 13 shows the ¹NMR spectra in D₂SO₄/D₂O of 3′ (top), 4 (middle) and 4+5 (bottom).

FIG. 14 shows the ¹NMR spectra in D₂SO₄/D₂O of 4+5.

FIG. 15 shows the ¹NMR spectra in D₂SO₄/D₂O of 4+6.

FIG. 16 shows the ¹NMR spectra in D₂SO₄/D₂O of 4+7.

FIG. 17 shows the ¹NMR spectra in D₂SO₄/D₂O of 4+8.

FIG. 18 shows the ¹NMR spectra in D₂SO₄/D₂O of 4+9.

FIG. 19 depicts thermogravimetric analyses plots for the MOFs of the invention.

FIG. 20 depicts first-derivative thermogravimetric analyses plots for the MOFs of the invention.

FIG. 21 depicts first-derivative thermogravimetric analyses plots for 4+6 evacuated at 150° C. (top) then resolvated with CHCl₃ (bottom).

FIG. 22 depicts CO₂ isotherms at 273K. Desorption curves are omitted for clarity.

FIG. 23 depicts H₂ isotherms at 77K. Desorption curves are omitted for clarity.

FIG. 24 depicts H₂ isotherms at 77K and 87K (black squares) and virial equation fits (red line) for 3′ (I).

FIG. 25 depicts H₂ isotherms at 77K and 87K (black squares) and virial equation fits (red line) for 4 (II).

FIG. 26 depicts heats of adsorption (ΔH_(ads)) for H₂ in 3′(blue) and 4 (black).

FIG. 27 depicts adsorption isotherms of CO₂, N₂, and CH₄ in 3′, 4, and 4+9 at 298° K: (a) full pressure range, (b) low pressure range (CH₄ isotherms are omitted for clarity).

FIG. 28 shows ideal adsorption solution theory selectivities of (a) CO₂ over N₂, and (b) CO₂ over CH₄ for equimolar binary mixtures in 3′, 4, and 4+9 at 298° K.

FIG. 29 shows ideal adsorption solution theory selectivities of CO₂ over N₂ in 9 at different pressures and mixture compositions.

FIG. 30 depicts adsorption rates of CO₂ and N₂ in 4+9 at 298° K (at the 1^(st) adsorption points). mt is the amount adsorbed at time t, and me is the equilibrium amount adsorbed.

FIG. 31 depicts thermogravimetric analyses plots for 3 and 4 of the invention.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

The invention relates to a new tetracarboxylic acid species (4,4′,4″,4′″-benzene-1,2,4,5-tetrayl-tetrabenzoic acid, 2), and salts thereof, as shown in Scheme 1. A deprotonated 2 was found to be a) an unusually shaped molecule, resisting formation of catenated MOFs, b) a tetra-topic building block, producing robust frameworks, and c) a nonplanar moiety, producing a 3D framework. These three characteristics favor the formation of comparatively large cavities, a desirable feature for post-assembly functionalization.

Solvothermal reaction of 2 and Zn(NO₃)₂.6H₂O in DMF at 80° C. for 24 hours afforded in high yield a MOF (3) having the framework formula [Zn₂(2)(DMF)₂]_(n) [DMF=dimethylform-amide] (Scheme 1, FIGS. 6 and 7), wherein n can range from about 10—about 100, and preferably from about 25—about 100. X-ray analysis of a single crystal of 3 revealed a non-catenated structure in which the framework nodes consist of Zn(II)₂ units coordinated by the carboxylates of 2 in paddlewheel fashion. Notably, the strut twists sufficiently to create a true 3D, rather than layered-2D, framework Importantly, the axial sites of the Zn(II)₂ units are ligated by solvent molecules.

Thermogravimetric analysis (TGA) of 3 revealed mass losses at about 100° C. and 175° C., assigned to free and coordinated DMF, respectively; no further mass loss occurs until 425° C. (FIG. 7A). Heating 3 under vacuum at 100° C. allows for selective removal of non-coordinated DMF, while heating under vacuum at 150° C. removes all solvent molecules. The partially and fully evacuated MOFs are designated, respectively, 3′ and 4. Void volumes from PLATON¹⁷ for 3′ and 4 are 53 and 65%, respectively. Follow up TGA experiments (FIG. 7A) show that 4 can be fully resolvated, while powder X-ray diffraction (PXRD) shows that the resolvated form retains crystallinity.

The thermal lability of coordinated DMF should permit its replacement by other ligands. Samples of 3 were converted to 4 and immersed for 24 hours in CH₂Cl₂ solutions of each of several candidate pyridine ligands (py-R, 5-9). Following an extensive washing, soaking, and drying protocol designed to remove solvent and free ligands, each of the putative py-R-modified MOFs was dissolved in D₂SO₄/D₂O. ¹H NMR measurements established the retention of py-R ligands (see example in FIG. 8).

In each case, proton peak integrations were consistent with complete derivatization of Zn(II) nodes and formation of the desired cavity-modified species, [Zn₂(2)(py-R)₂]_(n), wherein n is about 10—about 100, and preferably about 25—about 100. TGA measurements of rinsed and dried samples provided compelling support for coordinative (as opposed to sorptive) binding of the various py-R. As shown in FIG. 7B, the pyridines bind to the zinc sites more strongly than does DMF, with temperatures for dissociation ranging from ˜260 to ˜375° C. Finally, TGA measurements with resolvated samples established that the modified MOFs retain high porosities.

CO₂ adsorption (T=273K) was used to determine the accessible surface areas and pore volumes of the original and cavity-modified MOFs (See Table 2 and FIG. 22). The areas range from 310 to 1370 m²/g; the volumes range from 0.106 to 0.404 cm³/g, with the volume for the “empty cavity” MOF (4) being the largest. With these results in hand, the sorption measurements were extended to molecular hydrogen. At 77 K and 1 atm, 4 displays reasonably high H₂ uptake:¹⁸ 2.2% at 1 atm—roughly double the uptake by 3′. The difference can be attributed to the greater surface area for 4, as well as greater heats of adsorption (presumably due to open metal sites).

FIG. 10 summarizes hydrogen uptake data for the “empty cavity” MOF and the six cavity-tailored variants. At 77 K and 1 atm. the range of gravimetric loadings for these otherwise identical compounds spans a rather remarkable factor of four. Consistent with expectations from recent computational studies,¹⁹ the variations correlate well with both surface area and pore volume. While illustrating a relatively simple case (cryogenic H₂ uptake), the correlations clearly point to the potential for node-based, post-assembly modification for systematically altering sorption properties.

In another embodiment, cavity modification of 4 substantially altered the selectivity of the MOF for CO₂ versus methane. The adsorption in MOFs 3′, 4, and 4+9 were compared. Single-component adsorption isotherms for CO₂, N₂, and CH₄ were measured experimentally in all three MOFs. Then, from the pure-component isotherms, the selectivities for CO₂/N₂ and CO₂/CH₄ mixtures were calculated using ideal adsorbed solution theory (IAST)²⁰.

FIG. 27 shows the adsorption isotherms of CO₂, N₂, and CH₄ at 298 K up to 18 bar, measured volumetrically on evacuated samples of 3, 4, and 5. In each sample, CO₂ is the most strongly adsorbed molecule due to its large quadrupolar moment. Also, CH₄ shows stronger adsorption than N₂ as already reported in all known sorbents. This is attributed to the higher polarizability of CH₄ (26×10⁻²⁵ cm⁻³) VS. N₂ (17.6×10⁻²⁵ cm⁻³). Measurement of N₂ isotherms for any of the three MOFs at 77 K could not be made, but the materials did take up N₂ at 298 K. This suggested that the pores of 3′, 4, and 4+9 may be close to the kinetic diameter of N₂ (3.64 Å). For such tightly constricted pores, a likely explanation was that N₂ molecules cannot enter the pores at 77 K due to large diffusional resistances, but at 298 K the additional thermal energy allows the molecules to overcome these resistances.

None of the isotherms in FIG. 27 showed saturation at 18 bar. For all gases, the order of the adsorbed amounts around 18 bar was as follows: 4>3′>4+9 (FIG. 2A). At low pressures, 4 again showed the highest adsorption of the three MOFs for all three gases (FIG. 2 b), presumably due to strong adsorption on the open-metal sites rather than the larger surface area of 4. At low loading, the py-CF₃-modified MOF 4+9 adsorbs more CO₂ than 3 at 298 K, but less N₂ and CH₄.

The selectivities of CO₂/N₂ and CO₂/CH₄ binary mixtures were predicted from the experimental single-component isotherms using IAST. FIGS. 28 a and 28 b present the predicted selectivities for equimolar CO₂/N₂ and CO₂/CH₄ mixtures in 3′, 4, and 4+9 as a function of total bulk pressure. The most remarkable point of FIG. 28 was the high CO₂/N₂ selectivity (˜42) of 4+9 at low pressure. Throughout the entire pressure range, 4+9 exhibited larger CO₂/N₂ and CO₂/CH₄ selectivities than 3′ and 4. The following explanations were surmised: a) first, the highly polar —CF₃ groups in 5 should be more attractive to CO₂ (large quadrupole moment, 13.4 C m²) than N₂ (smaller quadrupole moment, 4.7 C m²) or CH₄ (nonpolar); and b) second, the more constricted pores of 4+9 should enhance the selectivity of the more strongly adsorbed CO₂ over N₂ and CH₄ due to the increased potential.

FIG. 29 shows the CO₂/N₂ selectivities in 4+9 at different pressures and different mixture compositions predicted by IAST. The selectivity increased with decreasing pressure. Also, the selectivity increased as y_(N2) approached unity, but at zero coverage it did not depend on the gas composition. For the case of y_(N2)=0.85, which is a typical composition for flue gas from power plants, the selectivity was in the range of 25-45. In addition, the selectivity was high (30-37), at or slightly above atmospheric pressure, the pressure regime of interest for removing CO₂ from flue gas. For these conditions, the selectivity of 4+9 was higher than that of Cu—BTC (20-22 as predicted by molecular simulation), the largest previously reported for MOFs.²¹ Moreover, these selectivities were considerably higher than the experimental CO₂/N₂ selectivities reported for zeolite and carbon adsorbents under similar conditions: zeolite 4A (19), zeolite 13× (18), activated carbon (15).

For PSA processes, the kinetics and reversibility of adsorption were also important. Adsorption of CO₂ was found to be completely reversible in 4+9 (FIG. 22), and a graph of the time evolution for CO₂ and N₂ adsorption in 4+9 at the first point of the isotherms (0.25 atm and 298 K) showed that the adsorption rate of CO₂ is much faster than that of N₂ (FIG. 30). Thus, selectivity of CO₂ over N₂ would increase even more if the adsorption kinetics were considered in addition to the adsorption equilibria. The fast and reversible adsorption of CO₂ in 4+9, along with the high selectivity, indicated that this material is an attractive candidate for the adsorptive separation of CO₂ from N₂.

EXAMPLES OF THE INVENTION

General Information.

Starting materials were purchased from Sigma-Aldrich (ACS grade) and used without further purification unless otherwise noted.

Thermogravimetric analyses (TGA) were performed on a Mettler-Toledo TGA/SDTA851e. Powder X-ray diffraction (PXRD) patterns were recorded with a Rigaku XDS 2000 diffractometer using nickel-filtered Cu Kα radiation (λ=1.5418 Å). Adsorption isotherms were measured with an Autosorb 1-MP from Quantachrome Instruments. ¹H NMR and ¹³C NMR were done on a Varian Inova 500 spectrometer at 500 MHz and 125 MHz respectively. Single crystals were mounted on a Bruker SMART CCD 1000 diffractometer equipped with a graphite-monochromated MoKa (λ=0.71073 Å) radiation source in a cold nitrogen stream.

Example 1

Synthesis of 1: 200 ml of (1M in THF, 200 mmol) p-tolylmagnesium bromide was added under nitrogen to a flask containing 5 g of benzene hexabromide (9.07 mmol). The mixture was stirred at room temperature for 15 hours (gray suspension). The reaction was quenched with ice followed by 6M HCl. The mixture was extracted with hexanes (3× 250 ml). The organics were combined and the solvent was removed via rotary evaporation. The solid was then washed with hexanes and collected by filtration. Isolated yield: 2.8 g, 70%. ¹H NMR (CDCl₃): δ 2.32 (s, 12H), 7.04 (d, 8H), 7.12 (d, 8H), 7.45 (s, 2H). ¹³C NMR (CDCl₃): δ21.4, 128.9, 130.0, 133.3, 136.4, 138.4, 139.5.

Example 2

Synthesis of 2: 4 g of 1 was placed in a teflon lined vessel. 45 ml of water and 15 ml of HNO₃ were then added. The vessel was sealed and heated at 180° C. for 24 hrs. The resulting solid was collected by filtration and washed with THF/CHCl₃. Isolated yield: 3.8 g, 75%. ¹H NMR (CDCl₃): δ 7.40 (d, 8H), 7.60 (s, 2H), 7.80 (d, 8H), 10.00 (s, 4H). ¹³C NMR (CDCl₃): δ129.9, 130.7, 133.1, 135.4, 139.9, 146.3, 192.0.

Example 3

Synthesis of 3: X-ray quality single crystals of 6 were obtained upon heating Zn(NO₃)₂.6H₂O (20 mg, 0.067 mmol), 2(10 mg, 0.018 mmol) in 1 ml of DMF at 80° C. for 24 hours. Isolated yield: 50% yield based on 2. This procedure can be scaled up using the same solution concentrations.

Example 4

Single crystal X-ray diffraction: Single crystals of 3 were mounted on a BRUKER APEX2 V2.1-0 diffractometer equipped with a graphite-monochromated MoKa (λ=0.71073 Å) radiation source in a cold nitrogen stream. All crystallographic data were corrected for Lorentz and polarization effects (SAINT). The structures were solved by direct methods and refined by the full-matrix least-squares method on F² with appropriate software implemented in the SHELXTL program package. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were added at their geometrically ideal positions. Most of the solvent molecules occupying the pores were severely disordered, which hindered satisfactory development of the model; therefore, the SQUEEZE routine (PLATON) was applied to remove the contributions of electron density from disordered solvent molecules. The outputs from the SQUEEZE calculations are shown in Table 1.

TABLE 1 Crystal data and structure refinement for 3 3 empirical formula C₂₀ H₁₆ N O₅ Zn formula weight 415.71 crystal color, habit colorless, tabular crystal dimensions (mm³) 0.151 × 0.105 × 0.032 crystal system orthorhombic space group Imma a (Å) 21.6546 (16) b (Å) 30.901 (2) c (Å) 9.2945 (8) α (deg) 90 β (deg) 90 γ (deg) 90 V (Å³) 6219.3 (8) 8 Z ρ (calcd, g/cm³) 0.888 μ (cm⁻¹) 0.808 goodness-of-fit on F² 0.838 0.1009 R R_(w) 0.2530

Example 5

Synthesis of 3′: crystals of 3 were evacuated while heating at 100° C. for 24 hours.

Example 6

Synthesis of 4: crystals of 3 were evacuated while heating at 100° C. for 12 hours then 150° C. for 12 hours.

Example 7

Synthesis of Modified 4: 50 mg of 4 was soaked for 24 hours in a solution of 3 ml of CHCl₃ and 1 ml of the pyridine derivative. The solid was filtered and evacuated while heating at 150° C. for 12 hours.

Example 8

¹H NMR of Modified MOF: 5 mg of the modified MOF was dissolved in D₂SO₄/D₂O. NMR spectra were obtained after the solid completely dissolved (See FIGS. 8 and 12-18).

Example 9

Adsorption measurements: Samples of known weight evacuated at the appropriate temperature under 10⁻⁵ torr dynamic vacuum for 24 hours on an Autosorb 1-MP from Quantachrome Instruments prior to gas adsorption measurements. The evacuated sample was weighed again to obtain the sample weight.

TABLE 2 H₂ uptake, surface areas, and pore volumes H₂-uptake Surface Area Pore Volume MOF (wt %) (m²/g) (cm³/g) 3′ 1.20 796 0.244 4 2.20 1366 0.404 4 + 5 1.24 709 0.214 4 + 6 0.59 370 0.132 4 + 7 0.57 309 0.106 4 + 8 1.04 473 0.165 4 + 9 0.57 388 0.131

Example 10

Isosteric heat of adsorption: The hydrogen isotherms obtained at 77 and 87 K were fit to the following virial equation: (Czepirski, L.; Jagiello, J., Chem. Eng. Sci. 1989, 44, 797.

${\ln \; p} = {{\ln \; N} + {\frac{1}{T}{\sum\limits_{i = 0}^{m}{a_{i}N^{i}}}} + {\sum\limits_{i = 0}^{n}{b_{i}N^{i}}}}$

The heats of adsorption of 3 and 4 were calculated from the fitting parameters in the following equation:

${q_{st}(N)} = {{- R}{\sum\limits_{i = 0}^{m}{a_{i}N^{i}}}}$

As demonstrated, a robust, non-catenated, and permanently microporous metal-organic framework (MOF) material has been synthesized by combining a new representative nonplanar ligand, 4,4′,4″,4′″-benzene-1,2,4,5-tetrayl-tetrabenzoic acid, with a Zn(II) source under solvothermal conditions. The new material features cavities that are readily modified via activation and functionalization of framework nodes. Investigation of the “empty cavity” version of the material and six cavity-modified versions reveals that modification can substantially modulate the MOF's internal surface area and pore volume. The resulting tailored cavities show differing degrees of uptake of molecular hydrogen under cryogenic conditions—an observation that may foreshadow a range of other applications, including cavity tuning of chemical catalysis and chemical separations.

Specifically, experimental isotherms and IAST calculations have shown that the MOFs of the invention are a promising material for CO₂/N₂ separations. In addition, they provide preliminary insight into the factors of most importance for adsorption selectivity of CO₂, N₂, and CH₄ mixtures in MOFs. Post-synthesis modification of MOFs by replacing coordinated solvent molecules with highly polar ligands or ligands featuring other chemical functionalities may be a powerful method for generating new sorbents for other difficult separations.

Thus, without limitation, the present invention can be utilized in the context of gas storage, gas/small molecule separations, gas/small molecules sensing, chemical catalysis and chemical protection.

Various aspects and features of this invention can be considered in the context of the following references, as enumerated above.

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1. A compound of the formula

and salts thereof.
 2. A metal-organic framework building block comprising a tetratopic carboxylic acid and a metal, the metal-organic building block being robust, non-catenated, 3-dimensional, and permanently microporous.
 3. The metal-organic framework building block of claim 2 wherein the tetratopic carboxylic acid comprises a phenyl ring core, substituted at the 1-, 2-, 4-, and 5-positions with substituted phenyl ring spacers.
 4. The metal-organic framework building block of claim 3 wherein the carboxylic acids are located at the 4-position of each phenyl ring spacer, the carboxylic acids binding to the metal.
 5. The metal-organic framework building block of claim 2 wherein the tetratopic carboxylic acid is of the formula A

and the metal is Zn(II) source.
 6. The metal-organic framework building block of claim 5 having a formula [Zn₂(A)(B)₂]_(n), wherein n is about 25—about 100 and B is nothing or selected from R-pyridin-4-yl and DMF, where R is selected from H, methyl, ethyl, ethenyl and CF₃.
 7. The metal-organic framework building block of claim 6 wherein B is R-pyridin-4-yl and R is selected from H, methyl, ethyl, ethenyl and CF₃.
 8. A gas adsorption separation process characterized by selectively removing one or more gases from a gas mixture by contacting the gas mixture with a metal-organic framework building block of claim
 6. 9. The gas adsorption separation process of claim 8 wherein CO₂ is selectively removed from the gas mixture.
 10. The gas adsorption separation process of claim 9 wherein B of the metal-organic framework is R-pyridin-4-yl and R is selected from H, methyl, ethyl, ethenyl and CF₃.
 11. The gas adsorption separation process of claim 10 wherein R is CF₃.
 12. A process for the preparation of a metal-organic framework of formula [Zn₂(A)(B)₂]_(n), wherein n is about 25—about 100, A is a tetratopic carboxylic acid, and B is nothing or selected from R-pyridin-4-yl and DMF, where R is selected from H, methyl, ethyl, ethenyl and CF₃, the process comprising: a) solvothermal reacting of the tetratopic carboxylic acid and Zn(NO₃)₂.6H₂O in DMF; b) optionally heating the product in a) under reduced pressure to remove the DMF; and c) optionally soaking the product in b) in a solution containing A to afford the metal-organic framework.
 13. The process of claim 12 wherein the tetratopic carboxylic acid is


14. The process of claim 13 wherein B is R-pyridin-4-yl and R is selected from H, methyl, ethyl, ethenyl and CF₃. 